192.com: provides its own people search based on electoral role data. You must sign up to use this service.
1st Locate: 1stlocate people search is a fee paying service that will help you find old friends, family, schoolmates, adoptee parents; even debtors.
Around.co.uk: is a free people finder specialising in the UK, USA, Australia, New Zealand, and also worldwide.
Forces Reunited: Another site from the people at Friendsreunited. Search and find people who were/are in the UK Armed forces.
Friends Reunited: Seek out your old classmates and find out what they are up to. Very famous and popular website to find old friends. Registration is free but you must pay to use the contact facility.
Peoplefinders.co.uk: Specialise in locating living people anywhere in the uk. Old friends and long lost relatives. Birth mothers and adopted children.
The Sikh Directory: has a worldwide directory for Sikhs to find other Sikhs.
UK Investigation Group: A commercial investigation company who aim to provide you with the address of almost anyone in the uk within an hour - so long as you know their current surname.
UK People Finder: is a commercial peoplefinder where you can place details of the person you are looking for and they will perform the investigation for you. They also offer a free service called messagematch for those people where the commercial search isn't viable.